Low density polyethylene (LDPE) having a high degree of long and short chain branching is generally produced in high pressure ethylene polymerisation units. Due to the long chain branching, LDPE exhibits low crystallinity and, when molten, a lower tensile strength and higher ductility than other known polyethylenes. These unique and desirable flow properties allow LDPE to be used for plastic film applications such as plastic bags and film wrap.
Copolymerisation of ethylene with vinyl acetate monomer (VAM) provides a copolymer known as ethylene-vinyl acetate (EVA). This polymer approaches the softness and flexibility of elastomeric materials, yet can be processed like other thermoplastics. The material generally has good barrier properties, good stress-crack resistance and high clarity and gloss. It also has high resistance to UV radiation. Furthermore, it has good heat sealing properties and can be used as a hot-melt adhesive. EVA has little or no odor and is competitive with rubber and vinyl products in many electrical applications. As it is virtually inert in the body, it is also used in biomedical engineering, for example in drug delivery applications. EVA can also be processed into a foam, which is used, for example, in the padding of sports equipment.
The high pressure ethylene polymerisation unit comprises a reactor wherein the polymerisation takes place under a pressure of from 50 to 300 MPa and at temperatures of from 100 to 400° C. It is a radical reaction, which can be triggered with initiators such as oxygen and peroxides. The reactor is either an autoclave reactor or a tubular reactor. Polymerisations carried out in an autoclave are adiabatic and hence no temperature control is required. However, in tubular reactors, care must be taken in order to control the reaction temperature or else thermal runaway may occur due to the exothermic nature of the reaction. To this end tubular reactors are made up of a plurality of interconnected double tubes. The inner central tube is where the reaction takes place and the outer, enveloping tube, also known as the cooling jacket, is where the cooling medium flows, regulating the temperature within the inner tube. The cooling jacket forms part of a larger cooling circuit, wherein the cooling medium can be cooled down again and sent back to the cooling jacket.
Temperature control increases productivity by keeping the reaction temperature under the decomposition limit. Usually, the cooling medium is water, in particular, demineralised water. The tubes are made of a special steel alloy designed to avoid violent ruptures resulting from the fatigue under the high pressures. Rather than rupturing, the tubes develop one or more small fissures. When these occur, the fissures steadily grow, creating a minor leakage of reaction medium into the cooling jacket. As the fissures expand, more and more polymer and monomer build up in the cooling jacket, causing a blockage in the circulation of the cooling water. Eventually the polymerisation temperature can no longer be efficiently controlled, which increases the risk of thermal runaway of the reaction. Once the leak is finally detected, the whole unit must be shut down in order to replace or repair not only the leaking reactor tube, but also the blocked cooling jacket. This is a heavy financial burden, not only in terms of equipment cost, but also in terms of lost production time. This problem can occur in any double tube within a high pressure ethylene polymerisation unit, whether in the tubular reactor or, for example, in the recycle systems of unreacted monomer. The higher the pressure within the tube, the higher the risk of a leak occurring.
Until now, the only available solution to the problem of detecting leaks in an ethylene polymerisation unit comprising a tubular reactor was to detect the presence of ethylene in the cooling water using a catalytic gas detector. However, these detectors cannot be installed online. Instead, the cooling water must be continuously sampled and analysed, which is not only an unnecessary waste, but also a complex process. In particular, during start-up of the polymerisation process, the cooling water in the first reactor zone is heated with hot water or steam, but the catalytic gas detector cannot withstand these conditions and is often damaged.
The tubular reactor may be divided into independent reaction zones, each with an independent cooling circuit. However, sampling cooling water from each cooling circuit and transferring it to each of the circuit's own independent condenser and gas detector is neither efficient, nor cost effective. Instead, sampling is usually combined through a common tube leading to a single condenser and detector. Unfortunately, it is then impossible to identify which reactor tube has incurred the leak, if ethylene is detected. Furthermore, the different pressures in each reactor zone cause preferential pathways in the cooling circuits. Even by carefully regulating the pressure valves, it is almost impossible to have a continuous simultaneous passage of gas from all the reactor zones to the detector.
It is thus an aim of the invention to improve the method of operating a high pressure ethylene polymerisation unit comprising a tubular reactor.
It is another aim of the invention to locate leaks in a high pressure ethylene polymerisation unit when copolymerising with a comonomer.
In addition, it is an aim of the invention to reduce sampling of cooling medium for detecting leaks in high pressure ethylene polymerisation units.
It is also an aim of the invention to reduce blockages in the cooling circuits comprised within high pressure ethylene polymerisation units.
It is yet another aim of the invention to reduce reactor downtime in case of a leak in high pressure ethylene polymerisation units.
It is a further aim of the invention to improve the detection of leaks in high pressure ethylene-vinyl acetate copolymerisation units.